1. Field of the Invention
This invention relates generally to a tough, durable and impact resistant continuous polymer coating. The polymer in accordance with this invention has very high strength and stiffness in a direction along the coating surfaces while being compliant in a direction normal to the coating surface. This permits the polymer to be very impact resistant and relates it to applications where high strength and/or impact resistance is desired in a moldable material. Since the polymer in accordance with the invention is continuous, it also relates to applications requiring high strength and/or impact resistance while being impermeable or transparent. More specifically, this invention relates to, but is not limited to, optical coatings for windows and domes (a window being defined herein as something between a system and the environment to protect the system from the environment) and particularly, but not limited to, coatings for infrared windows and domes, primarily, but not limited to, use on aircraft, and, more specifically to a polymer or plastic optical coating to protect infrared optics, particularly infrared windows and domes.
2. Brief Description of the Prior Art
To increase the survivability and operational capability of infrared windows and domes, particularly as used during flight and particularly for the 8 to 12 micrometer wavelength (infrared) region, protective coatings are required for rain, dust, sand and hail impact. The impact of these particles during flight (aircraft, missile, helicopter, etc.) on the window or dome erodes the window or dome, thereby reducing its strength and ability to transmit infrared or other energy radiations therethrough. This degradation can render the electro-optical sensor behind the dome or window inoperable or even damaged should the window or dome catastrophically fail.
Presently used prior art infrared windows and domes degrade in performance due to loss of transmission and strength due to environmental degradation, particularly due to erosion by rain, dust and sand particles at aerodynamic speeds.
Prior art solutions to this problem have involved the use of a protective coating on the infrared domes and windows. Due to the requirement that the protective coating be transparent in the wavelength region in which the window or dome operates (i.e., 8 to 12 micrometers, 3 to 12 micrometer, 3 to 5 micrometers, 1 to 12 micrometer, etc.), past and current efforts on protective coatings have concentrated on traditional inorganic materials, such as silicon, gallium phosphide, boron phosphide, diamond, germanium carbide, silicon nitride, silicon carbide, oxides, etc. to obtain the desired transparency. These coating have displayed high strength, high fracture toughness, high hardness and moderate to high elastic (Young's) modulus.
The general mechanical requirements of coatings for soft (rain) and hard (sand, hail, dust, etc.) particle impact protection of substrates are low hardness and high fracture toughness or strength with a high elastic modulus to reduce the strain induced in the substrate or a low elastic modulus to absorb the impact stress. Accordingly, the above-mentioned materials have shown only limited effectiveness in solving the problem of erosion due to particle impact and have been difficult to scale up in size. It is therefore apparent that other solutions to the problem were required which overcome or minimize those problems.
The above described problems were reduced by providing polymeric coatings for infrared windows and domes which were infrared transparent polymers with low hardness and high strength as set forth in the above-mentioned application. These polymers absorb and distribute the stresses of the impacting particles, thereby protecting the underlying infrared optics, primarily infrared windows and domes and primarily, but not limited to, the 8 to 12 micrometer wavelength range.
The polymeric infrared transmitting coating described in the above mentioned application has been found to be very effective in providing the required protection for infrared windows and domes against particles which do not set up stress waves in the plane of the window or dome surface. Such materials are inexpensive and readily available in films which can be placed on the exterior surface of an infrared window or dome. Polymers had been overlooked prior to the above noted application for use as infrared optical protective coatings, apparently due to their well-known absorption bands throughout the infrared range. These bands are the intrinsic molecular vibrational absorption due to the constituents of the polymer (i.e., C--H stretching, bending modes). However, on detailed analysis of the infrared spectra of various polymers, some are highly transparent in, for example, the 8 to 12 micrometer region, where considerable interest and applications exist for electro-optical systems. These same 8 to 12 micron transparent polymers and copolymers, such as, for example, and not limited to, polyethylene, ethylene-octene copolymer, polyvinylpyrrolidene, poly(acenaphthylene), styrene/ethylenebutylene copolymer, poly(1-butene), poly(acrylic acid, ammonium salt), polyamide resin, ethylene/propylene copolymer and ethylene/propylene/diene terpolymer, possess low hardness, high strength and low elastic (Young's) modulus, making them candidates as particle impact and erosion resistant coatings for infrared windows and domes.
The polymers of choice in the above noted applications are those that provide infrared transmissivity in the desired wavelength range, such as, for example, 8 to 12 micrometers, low hardness less than about 50 kg/mm.sup.2, high strength in the range of 10,000 to 100,000 psi with a preferred value of greater than 20,000 psi and low elastic (Young's) modulus in the range of 0.2 to 3.times.10.sup.6 psi and preferably less than 0.5.times.10.sup.6 psi. Most polymers do not display transmissivity in the infrared range and it has been generally believed that polymers in general do not display such transmissivity and are absorbent to infrared energy. In those cases where the polymers provide the desired optical properties but fail to provide the desired mechanical properties, copolymers of the optically desirable polymers and other polymers which provide the desired mechanical properties can be formulated to provide a compromise which still presents the critical properties in the desired ranges. The ethylene-octene copolymer is an example of such copolymer. Additional copolymers or terpolymers are desirable to optimize the optical transparency and the mechanical and thermal properties, particularly strength and thermal stability. Candidates include polyethylene, ethylene-octene copolymer, polyvinylpyrrolidene, poly(acenaphthylene), styrene/ethylenebutylene copolymer, poly(1-butene), poly(acrylic acid, ammonium salt), polyamide resin, ethylene/propylene copolymer and ethylene/propylene/diene terpolymer, which are infrared (8 to 12 micrometers) transparent and neoprene, polyurethane, fluorelastomer, polycarbonate, polyether sulfone, polyether etherketone and polyacrylate which are very rain erosion resistant. Also, the copolymers can be tailored to provide vibrational modes of the atoms therein at frequencies outside of the optical frequency range of interest to possibly provide the desired transmissivity in the frequency range of interest.
The sheet of polymeric material is placed on the optical window or dome in any of many well known ways, such as, for example, by static or chemical bond with or without an intermediate "glue" layer, as required, spinning on, spraying on or cast on and allowed to set.
A problem with the above described polymeric sheet material is that it is compliant in both the plane normal thereto as well as in the plane thereof. Accordingly, though hard particles, such as sand, cause stress waves to propagate essentially only along the line of impact of the window therewith, which is normal to the plane of the polymeric material, rain additionally causes stress waves to propagate along the surface of the window in a direction essentially along the plane of the window and the polymeric covering thereon. These stress waves in the plane of the polymeric material cause a polymer coating of the type discussed with reference to the above noted copending application to move along the path of the stress waves or in the plane of the polymeric coating. Since the window or dome to which the coating is attached does not undergo such movement, there is a tendency for the polymer coating to separate or delaminate from the dome or window and eventually tear and thereafter possibly further be removed from the window. This causes a loss of the window or dome protection previously obtained from the coating.